This invention relates to internal combustion engine cylinder assemblies and their associated valve openings and poppet valves for the intake and exhaust of an air-fuel mixture.
The thermal efficiency and fuel consumption of an internal combustion engine are very important factors in the overall cost of operation of the engine. As the cost of engine fuel increases, the emphasis on improving the fuel consumption of an engine also increases. Better fuel economy has become a major objective of research and development programs being carried on by major engine manufacturers today. Further effort is being targeted at obtaining a greater quantity of air into the cylinder assembly during the intake stroke which then can be utilized to either produce more power for the engine or lower the fuel consumption of the engine, or a combination of both.
Common methods of increasing the air quantity available in the cylinder assembly for combustion are supercharging the cylinder assembly through the use of mechanically driven blowers, and, more commonly, turbochargers, cooling the air by water to air, or air to air, heat exchangers, using quick opening cams to reduce the throttling loss through the intake valve, and so forth.
The variable that relates to the amount of charged air in the cylinder assembly is volumetric efficiency. Volumetric efficiency is a measure of the actual quantity of air in the cylinder assembly at the end of the intake process compared to the amount of air that could be in the cylinder assembly at normal atmospheric temperature and pressure.
Normally aspirated engines (non-supercharged) must necessarily have a volumetric efficiency of less than one hundred percent (100%) because of heat transferred to the air as it passes through the intake valve and enters the cylinder assembly and a pressure drop due to losses in passing through the narrow opening around the valve. Both the heat transfer and pressure drop due to the restricted valve opening reduce the quantity of air in each cylinder to less than that represented by atmospheric condition. The fuel consumption of diesel engines is usually improved by providing more excess air than required by the chemically correct mixture of air and fuel (stoichiometric mixture), however, any design feature that improves volumetric efficiency can be used to reduce fuel consumption in an internal combustion engine.
Providing an excess quantity of air also reduces noxious emissions. More complete combustion of the air-fuel mixture reduces the amount of residual hydrocarbons and carbon monoxide present in the exhausted mixture. More air in the cylinder assembly usually lowers the maximum temperature of combustion, thus reducing the amount of nitrogen oxide formed in the cylinder assembly which subsequently escapes in the exhaust gases.
In supercharged engines, the amount of air forced into the cylinder assembly generally exceeds the amount that could be present at normal atmospheric temperature and pressure so that the volumetric efficiency of this type of engine is usually greater than one hundred percent (100%). However, since the intake manifold pressure is increased to levels much above atmospheric by the supercharger, the pressure loss in the air passing through the intake valve is increased substantially. If the throttling loss across the valve can be reduced by valve design, then the supercharger pressure can be lowered. This in turn would lower the power absorbed in driving the supercharger from the engine and reduce the engine fuel consumption for the same power output of the engine.
The throttling loss through the intake valve is even more important in turbocharged engines than in supercharged engines. As the combusted mixture is exhausted through the exhaust valve, another pressure loss occurs. The pressure loss across the exhaust valve in such an engine is of more concern because it also affects the amount of exhaust gas energy available to drive the turbocharger turbine. When the exhaust valve starts to open, there is a very high residual pressure existing in the cylinder assembly near the end of the power-producing expansion stroke. Since the prevailing pressure in the exhaust manifold is much lower, there is a large pressure drop across the exhaust valve during the first part of the opening of the valve. This represents a large loss of energy in the exhaust gas that cannot be recovered and that cannot be used by the turbocharger turbine. Later in the exhaust stroke, the upward motion of the piston forces the residual exhaust gases out through the open exhaust valve into the exhaust manifold with a further loss in energy due to the small flow-through area around the valve head. The overall result of these conditions causes a substantial reduction in the pressure available to the turbocharger turbine from the pressure that was present in the cylinder assembly as the exhaust valve begins to open and as the exhaust gases flow out through the valve opening and expand into the exhaust manifold. The turbocharger turbine is driven by a pressure drop across its blading to produce power and higher pressures at the turbine inlet produce more turbine power and higher turbine outputs. Nozzles are provided in the turbine casing to recover valve pressure losses and raise the gas pressure from the exhaust manifold and obtain satisfactory power levels from the turbocharger turbine. The nozzles, however, introduce back pressure on the cylinders and increase engine pumping losses. If more of the gas pressure of the engine cylinders can be preserved prior to the turbocharger turbine nozzle, then the turbine nozzle area can be increased. The result of larger nozzle area in the turbocharger turbine is a lower average back pressure on the engine cylinder assemblies, a lower engine pumping loss, and a reduction in engine fuel consumption.
Current internal combustion machines utilize intake and exhaust valves that have round peripheries. Two valves per cylinder, one intake and one exhaust, are commonly used. However, many modern engines use a four valve per cylinder assembly configuration to increase the total flow-through area across the valves and produce the benefits of reduced valve losses that have been previously described. An increase in flow area of approximately fifty to sixty percent can be expected in a given cylinder assembly by using four valves versus two valves; however, the use of four valves rather than two valves significantly increases the cost of the valve assembly and valve operating mechanism. A recent study by a Japanese engine manufacturer focused on the optimization of multi-valve engine designs; the study analyzed four, five, six and seven-valve designs and concluded the five-valve design to be most efficient, further complicating the valve assembly and valve mechanism. However, the study considered only conventional internal combustion engines having valves with round peripheral shapes. The use of round valve heads in a round cylinder assembly, however, does not permit the use of maximum available flow-through area in either the intake or exhaust port and also does not optimize the volumetric efficiency of the cylinder assembly.